In next generation of high-speed wired and wireless networks, wavelength division multiplexed passive optical network (WDM-PON) and Radio-over-Fiber (RoF) system have been considered to be one of the most promising candidates for future wired and wireless access networks, respectively. As compared to conventional fiber to the home/premises (FTTH/P) systems, the WDM-PON provides much higher bandwidth and lower losses, latency, and jitters. The RoF system fulfils the next generation broadband wireless access network requirement of very high frequency operation, typically in the GHz ranges.
FIG. 1A show a schematic diagram illustrating a WDM-PON 100. In FIG. 1A, the downstream signal of each user is modulated with a specific wavelength at the optical line terminal (OLT) 102 located at the central office (CO). The wavelengths are then combined and sent together to the remote node (RN) 104 via a feeder fiber 106. After being de-multiplexed at the RN 104, the modulated optical signals 108 are sent to respective optical network units (ONUs) 110, where they are converted to electrical signals.
FIG. 1B shows a schematic diagram illustrating a RoF system 112. In the RoF system 112, the downstream signals follow a process similar to that of the WDM-PON 100 in that the downstream signal of each user of the RoF system 112 is modulated with a specific wavelength at the OLT 114 and that the wavelengths are then combined and sent together to the RN 116 via a feeder fiber 118. After being de-multiplexed at the RN 116, the modulated optical signals 120 are sent to respective base station (BS) 122, where they are converted to electrical signals. At each BS 122, the electrical signal is up-converted to RF band before being broadcasted.
Both the WDM-PON and the RoF schemes as shown in the WDM-PON 100 and the RoF system 112 of FIGS. 1A and 1B, respectively, share a similar concept of having signals modulated and transmitted to the receiving end by photonic means. As a result, the WDM-PON and the RoF may be integrated to deliver wired and wireless services.
It has been observed that a number of unique obstacles emerges with such an integration and simultaneously providing ultrahigh speed wired and wireless access services leads to several issues. Further, enormous initial capital and operational expenditures have hindered the wide deployment of such technology.
For example, one obstacle is to optimize the utilization of the optical spectrum for both wired and wireless signals. It has been proposed to use time-division multiplexing (TDM) in WDM-PON, polarization modulation, and a combination of both methods. However, TDM does not provide a solution for future bandwidth's requirement and polarization modulation/control proves to be complicated.
It has also been suggested that baseband wired signal and RF wireless signal may be modulated into the same optical carrier, and then low-speed and high-speed photo detectors may be used to separate the data at a receiver. However, such a method significantly increases the system cost.
Using optical carrier re-modulation may have been shown to be a more cost-effective method by utilizing the optical wavelength. However, more than often the system's performance is severely limited by crosstalk.
For RoF systems, the RF wireless data is conventionally sent downstream using the subcarriers and the optical carrier is for upstream re-modulated data. However, for an integrated system, using the subcarriers for wireless data is not possible. Moreover, for wireless signal at RF, the modulated optical signals required expensive and high loss optical devices for modulation and detection. For high speed wireless systems, complex modulation method such as orthogonal frequency division multiplexing (OFDM) is referred.
RoF systems for wireless access or WDM-PONs for wired access have been or are currently being investigated. Although integration of wired and wireless access over a single WDM-PON reveals potentials in providing network access improvements, such an integrated system also leads to a number of issues. A system dealing with at least all of the above-mentioned obstacles and issues have yet to be reported.
Thus, there is a need to provide an integrated access network that would have a simple structure, provide high (large) bandwidths and is cost effective; thereby seeking to address at least the above-mentioned problems.